Gamma Ray
Gamma Ray Gamma radiation, also known as gamma rays or hyphenated as gamma-rays (especially in astronomy, by analogy with X-rays) and denoted as γ, is electromagnetic radiation of high frequency (very short wavelength). Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei (gamma decay). Important natural sources are also high-energy sub-atomic particle interactions resulting from cosmic rays. Such high-energy reactions are also the common artificial source of gamma rays. Other man-made mechanisms include electron-positron annihilation, neutral pion decay, fusion, and induced fission. Some rare natural sources are lightning strike and terrestrial gamma-ray flashes, which produce high energy particles from natural high-energy voltages. Gamma rays are also produced by astronomical processes in which very high-energy electrons are produced. Such electrons produce secondary gamma rays by the mechanisms of bremsstrahlung, inverse Compton scattering and synchrotron radiation. Gamma rays are ionizing radiation and are thus biologically hazardous. A classical gamma ray source, and the first to be discovered historically, is a type of radioactive decay called gamma decay. In this type of decay, an excited nucleus emits a gamma ray almost immediately on formation, although isomeric transition can produce inhibited gamma decay with a measurable and much longer half-life. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium.[1][2] Villard's radiation was named "gamma rays" by Ernest Rutherford in 1903.[3] Gamma rays typically have frequencies above 10 exahertz (or >1019 Hz), and therefore have energies above 100 keV and wavelength less than 10 picometers, less than the diameter of an atom. However, this is not a hard and fast definition but rather only a rule-of-thumb description for natural processes. Gamma rays from radioactive decay commonly have energies of a few hundred keV, and almost always less than 10 MeV. On the other side of the decay energy range, there is effectively no lower limit to gamma energy derived from radioactive decay. By contrast, energies from astronomical sources can be much higher, ranging over 10 TeV (this is far too large to result from radioactive decay).[4] The distinction between X-rays and gamma rays has changed in recent decades. Originally, the electromagnetic radiation emitted by X-ray tubes almost invariably had a longer wavelength than the radiation emitted by radioactive nuclei (gamma rays).[5] Older literature distinguished between X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10−11 m, defined as gamma rays.[6] However, with artificial sources now able to duplicate any electromagnetic radiation that originates in the nucleus, as well as far higher energies, the wavelengths characteristic of radioactive gamma ray sources vs. other types, now completely overlaps. Thus, gamma rays are now usually distinguished by their origin: X-rays are emitted by definition by electrons outside the nucleus, while gamma rays are emitted by the nucleus.[5][7][8][9] Exceptions to this convention occur in astronomy, where high energy processes known to involve other than radioactive decay are still named as sources of gamma radiation. A notable example is extremely powerful bursts of high-energy radiation normally referred to as long duration gamma-ray bursts, which produce gamma rays by a mechanism not compatible with radioactive decay. These bursts of gamma rays, thought to be due to collapse of stars called hypernovas, are the most powerful single events so far discovered in the cosmos. Naming conventions and overlap in terminology In the past, the distinction between X-rays and gamma rays was based on energy (or equivalently frequency or wavelength), with gamma rays being considered a higher-energy version of X-rays. However, modern high-energy (megavoltage) X-rays produced by linear accelerators ("linacs") for megavoltage treatment in cancer radiotherapy, usually have higher energy (typically 4 to 25 MeV) than do most classical gamma rays produced by radioactive gamma decay. Conversely, one of the most common gamma ray emitting isotopes used in diagnostic nuclear medicine, technetium-99m, produces gamma radiation of about the same energy (140 keV) as produced by a diagnostic X-ray machine, and significantly lower energy than therapeutic photons from linacs. Because of this broad overlap in energy ranges, the two types of electromagnetic radiation are now usually defined by their origin: X-rays are emitted by electrons (either in orbitals outside of the nucleus, or while being accelerated to produce Bremsstrahlung-type radiation), while gamma rays are emitted by the nucleus or from other particle decays or annihilation events. There is no lower limit to the energy of photons produced by nuclear reactions, and thus ultraviolet and even lower energy photons produced by these processes would also be defined as "gamma rays".10 In certain fields such as astronomy, higher energy gamma and X-rays are still sometimes defined by energy, since the processes which produce them may be uncertain. Occasionally, high energy photons in nature which are known not to be produced by nuclear decay, are nevertheless referred to as gamma radiation. An example is "gamma rays" from lightning discharges at 10 to 20 MeV, which are known to be produced by the Bremsstrahlung mechanism.11 Another example is gamma ray bursts, which are named historically, and now known to be produced from processes too powerful to involve simple collections of atoms undergoing radioactive decay. A few gamma rays known to be explicitly from nuclear origin are known in astronomy, with a classic example being that of supernova SN 1987A emitting an "afterglow" of gamma-ray photons from the decay of newly-made radioactive cobalt-56 ejected into space in a cloud, by the explosion. However, many gamma rays produced in astronomical processes are produced not in radioactive decay or particle annihilation, but rather in much the same manner as the production of X-rays, but simply using electrons with higher energies. Astronomical literature tends to write "gamma-ray" with a hyphen, by analogy to X-rays, rather than in a way analogous to alpha rays and beta rays. This notation tends to subtley stress the non-nuclear source of many astronomical gamma rays.